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January 2013

Spotlight Summary by Anders Tegtmeier Pedersen

Remote sensing of wind in general and the use of Doppler wind lidars in particular keep finding more and more applications. One example is runway surveillance in airports to ensure safe take-offs and landings. Another one is of course within the wind energy industry, where their use has increased dramatically in recent years, and several commercial lidars have consequently found their way to the market. Doppler wind lidars can be seeded by either CW or pulsed lasers leading to different advantages and drawbacks. CW seeded lidars are characterised by their simple construction and their ability to measure at very close ranges, but because range scanning is achieved by changing the focus of the laser beam, their spatial resolution drops dramatically at long ranges. Pulsed lidars, on the other hand, offer inherent range scanning together with a fixed spatial resolution determined by the pulse length, but are in general more complicated than CW systems and are blind at short ranges.

In this Optics Express article, Brinkmeyer and Waterholter present a concept potentially able to bridge the gap between CW and pulsed lidars using low-coherence reflectometry. The result is a continuously emitting lidar with no moving parts, inherent range scanning, and a spatial resolution that can be even smaller than that of conventional pulsed lidars. Contrary to conventional Doppler lidars, this system requires a broad-banded laser source which is achieved by phase modulation of the output of a conventional laser diode. The modulation applied, however, is not just any random sequence but has been carefully predetermined with the aim of achieving a Gaussian optical spectrum. The use of such laser source has two clear advantages; firstly, the Gaussian spectrum falls off faster than the Lorentzian spectrum of many natural lasers leading to a more tightly confined measurement volume for the lidar, and secondly, since the phase is known, the measurement range can be shifted numerically in the post-processing and the wind speed at any given range can in principle be deduced from one single measurement. Since the measurement range in this way can be varied in the post processing, there is no need for changing the focus of the output laser beam and thus the entire system can be realised with no moving parts.

The authors have investigated the proposed system thoroughly through simulations of various scenarios regarding noise domains and ranges, and also experimentally tested the lidar in a laboratory environment with moving retroreflecting films acting as backscatter targets. The outcome of these tests clearly demonstrates the alleged features of the system with a constantly emitting laser and scanning of ranges without any moving parts. What still remains to be shown is whether the system can make the step from hard target measurements at short ranges in the laboratory to measurements of dispersed targets in the atmosphere at longer ranges and with much lower return signals.